This invention relates to thermal and pyrolytical analysis of samples having volatile components, nonvolatile, pyrolyzable components, and inorganic components by means of pyrolysis gas chromatography. More particularly, the invention relates to a simple, rapid, inexpensive and highly reproducible method for inserting a sample containing both volatile and nonvolatile, pyrolyzable components into a carrier gas stream flowing to a chromatographic column and for operating the associated chromatographic apparatus. In another aspect, the present invention relates to a method and apparatus for analyzing viscous, volatile liquids.
Pyrolysis is one of the oldest analytical techniques, dating back to the middle of the nineteenth century, still widely used by the analytical chemist today. The analytical techniques for pyrolytic studies of polymers has evolved from classical chemical methods, as disclosed for example by T. Midgley, Jr., et al., Journal of the American Chemical Society, Volume 51, page 1215 (1929), to instrumental methods. The modern instrumental methods employed in analytical studies are mass spectrometry, infrared spectroscopy, and gas chromatography. Gas chromatography, especially, has renewed the importance of pyrolysis as a tool in polymer analysis. Beginning in 1954, numerous studies on pyrolysis and gas chromatography were conducted, resulting in the publication of a large number of articles on pyrolysis gas chromatography, some of which are cited in "Pyrolysis Gas Chromatography Analysis of Rubbers and Other High Polymers", John Chih-An Hu, Analytical Chemistry, Volume 49, page 537, April, 1977.
The development of pyrolysis gas chromatography (PGC) as a precise analytical tool has been retarded by problems of interlab reproducibility and standardization. One approach to overcoming the prior difficulties in pyrolysis gas chromatography has been to use the Curie point pyrolyzer. Other researchers have suggested substituting photolysis for pyrolysis to improve reproducibility and standardization. Still others have suggested the use of sealed sample holders or the use of multicolumn chromatographic systems. Each of the prior art approaches has improved the performance of gas chromatographic methods but standardization and reproducibility are still significant problems to be overcome.
Variations in prior art PGC data have been attributed to several factors, such as variations in instrument design, pyrolyzer geometry, and temperature rise time. In addition to these factors, there are still other considerations that deserve special attention. When the sample to be pyrolyzed is a pure polymer or a compounded polymeric material with volatile constituents removed by extraction, reproducibility is relatively easy to achieve. However, complications usually arise when a compounded polymeric material in its original form, i.e., containing volatile materials, is analyzed. The prior art pyrolysis chromatogram of a compounded polymeric material in its original form is not a simple pyrogram but rather a compound pyrogram resulting from the inconsistent and nonreproducible superposition of a number of component chromatograms.
During pyrolysis, both vaporization and thermal degradation occur at the same time. If the injection port temperature of the chromatographic system approaches the boiling points of some but not all of the volatile constituents of a sample, some of the volatile constituents will be vaporized as soon as the sample is introduced into the chromatographic apparatus. Those vapors will then be developed into a chromatogram. The remaining volatile constituents will be vaporized by the initial heating of the pyrolysis probe heater after the probe heater is energized. The remaining vapors develop a chromatogram different from the vapors initially released. Moreover, part of the vapors released upon energization of the probe heater may undergo vapor phase pyrolysis to produce a pyrogram different from the chromatogram that would otherwise be produced from the nonpyrolyzed vapors. Finally, the nonvolatile constituent is pyrolyzed to develop yet a different pyrogram. All of the chromatograms and pyrograms thus developed, except the pyrogram of the nonvolatile constituent itself, are subject to variations depending on injection port temperature and post-injection waiting period (the time interval between sample insertion and pyrolysis). A logical prior art solution to these problems is to eliminate the volatile components by solvent extraction. Solvent extraction, however, is time consuming, requires large sample sizes and does not meet the quality assurance criteria of simplicity, rapidity, and inexpensiveness.
Accordingly, it is a broad object of the present invention to provide a standardized method of using pyrolysis gas chromatography that will yield reproducible results on a wide variety of materials when employing diverse kinds of chromatographic systems. It is another object of the present invention to provide analytical methods for testing materials such as commercial compounded polymeric materials, for example, rubbers and other high polymers, that meet the standards required by the aerospace industry quality assurance guidelines and provisions. It is a further object of the present invention to provide analytical methods utilizing pyrolysis gas chromatography that are accurate, rapid, simple, and inexpensive.
Another problem associated with prior art PGC is the introduction of oils and other viscous liquids into the gas chromatographic equipment. Previously, samples of oils and other viscous liquids are introduced into the injection port of the gas chromatographic equipment by one of three basic procedures. First the sample is dissolved in a suitable solvent to make a solution. The solution is thereafter injected by means of a syringe into the injection port through a septum covering the injection port. A second method is to place the sample in a glass tube and seal both ends of the tube. The sealed tube is then placed in a special apparatus, conventionally called a "solid sample injector" and thereafter is inserted through the injection port. The solid sample injector includes an apparatus for breaking the glass tube after it is in the injection chamber by pushing a plunger located on the exterior of the injection chamber. A third method for introducing oils and other viscous liquids into an injection chamber is to inject oils directly into the injection chamber through a septum covering the injection port by means of a syringe.
Both of the first and second procedures outlined above are tedious and time consuming. The third procedure is not workable under normal conditions because the viscous oils and other viscous liquids can hardly be drawn into the syringe in the first place and thereafter ejected from the syringe because of their viscosity. Moreover, at the elevated pressures employed in the gas chromatographic analysis of viscous liquids, normally on the order of 40 p.s.i.g., volatile liquid components tend to leak back through the septum, causing not only a loss of sample but perhaps precipitating serious health problems in the operator of the chromatographic equipment should any of the volatile components be toxic. See Sansone, E. B., Analytical Chemistry, Volume 49, page 670, April 1977.
Accordingly, it is another object of the present invention to provide a method of introducing viscous liquid samples into a gas chromatography apparatus that does not require sample preparation, that is simple and rapid to effect, that provides accurate and reproducible chromatograms, that requires a very small amount of sample, that is inexpensive and that is relatively safe for the operator of the chromatographic equipment.